1. Field of the Invention
The invention relates to an electrical voltage supply for providing a controlled output voltage.
2. Description of Related Art
Gabriel A. Rincon-Mora et al., “A Low-Voltage, Low Quiescent Current, Low Drop-Out Regulator”, IEEE Journal of Solid-State Circuits, Vol. 33, No. 1, January 1998 disclose a voltage supply of the above type that has a voltage source whose output voltage is independent of the load current. The voltage source is assigned to a closed-loop control circuit which has as an actuating element a field-effect transistor whose source-drain path connects a first connection of the voltage source to a first output connection of the voltage supply. A second connection of the voltage source is connected to a second output connection of the voltage supply. An electrical load may be connected to the output connections of the voltage supply. The gate of the field-effect transistor is connected to an actuation signal output of an operational amplifier, which serves as a closed-loop controller. A noninverting input of the operational amplifier is connected to the output voltage that is present at the output connections across a voltage divider, and a desired voltage signal is present at an inverting input of the operational amplifier. When a deviation between the output voltage and the desired voltage signal occurs, the operational amplifier changes the gate voltage that is present between the gate and the source of the field-effect transistor in such a way that the deviation is reduced. Since the field-effect transistor is controlled via the gate-source voltage and since the source is connected to an output connection of the voltage supply, it is also possible to achieve closed-loop control of the output voltage even when the output voltage of the voltage source is only somewhat larger than the output voltage of the voltage supply that is present at the load. As a result, when current is drawn at the output connections, only a slight power loss occurs at the field-effect transistor. This is particularly advantageous with portable electronic devices in which a disposable or rechargeable battery serves as the voltage source. When power is drawn from the voltage supply, though, it is disadvantageous if a voltage drop occurs at the internal resistance of the voltage source, which is always present, aside from that at the field-effect transistor. The voltage source therefore should be dimensioned in such a way that its source voltage is slightly above the desired output voltage at the maximum current draw. However, when the current draw is low, a small voltage drop on the internal resistance of the voltage source results. In order nevertheless to regulate the output voltage to the desired value a corresponding voltage drop must be produced at the field-effect transistor, which causes a high power loss and therefore a low efficiency. Reducing the internal resistance directly by configuring a plurality of available voltage sources in parallel and thereby increasing the efficiency is particularly impracticable if the individual voltage sources have different characteristics, for example different open-circuit voltages, which can result in quadrature-axis currents and therefore energy losses. Furthermore, the use of different voltage sources is not practicable for the same reason.
Therefore, the object of the invention is to provide a closed-loop-controlled electrical voltage supply that makes it possible to have high efficiency in a wide working range.